Update CONTRIBUTING.md (#2007)
Update CONTRIBUTING.md (#2007)

Adds instructions for setting GOPROXY.
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib